Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene is likely to increase, due to a developing shortage of propylene.
Thus, a process that uses higher alkenes instead of propylene as feed and coproduces higher ketones, such as cyclohexanone, rather than acetone may be an attractive alternative route to the production of phenols. For example, there is a growing market for cyclohexanone, which is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam and nylon 6.
It is known from U.S. Pat. No. 5,053,571 that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a catalyst comprising ruthenium and nickel supported on zeolite beta and that the resultant cyclohexylbenzene can be processed in two steps to cyclohexanone and phenol. The hydroalkylation reaction is carried out at a liquid hourly space velocity (LHSV) ranging from 1 to 100, a reaction pressure ranging from 100 to 1000 kPa, a hydrogen feed rate ranging from 0.2 to 6 mole per mole of feedstock per hour, and a reaction temperature ranging from 100 to 300° C.
In addition, U.S. Pat. No. 5,146,024 discloses that benzene can be reacted with hydrogen in the presence of carbon monoxide and a palladium-containing zeolite X or Y to produce cyclohexylbenzene, which can then be converted in high yield to phenol and cyclohexanone by autooxidation with subsequent acid treatment. The hydroalkylation reaction is carried out at a liquid hourly space velocity (LHSV) of the benzene feed of about 1 to about 100 hr−1, a total reaction pressure of about 345 to about 10,350 kPa, a molar ratio of H2 to benzene of about 0.1:1 to about 10:1, a molar ratio of carbon monoxide to H2 of about 0.01:1 to about 0.3:1, and a temperature of about 100 to about 250° C. Preferred operating conditions are said to include a LHSV of the benzene feed of about 5 to about 30 hr−1, a total reaction pressure of about 1,380 to about 4,830 kPa, a molar ratio of H2 to benzene of about 0.2:1 to about 1:1, a molar ratio of CO to H2 of about 0.02:1 to about 0.1:1, and a reaction temperature of about 140 to about 200° C.
U.S. Pat. No. 6,037,513 discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising at least one hydrogenation metal and a molecular sieve of the MCM-22 family. The hydrogenation metal is preferably selected from palladium, ruthenium, nickel, cobalt and mixtures thereof and the contacting step is conducted at a temperature of about 50 to 350° C., a pressure of about 100 to 7000 kPa, a benzene to hydrogen molar ratio of about 0.01 to 100 and a WHSV of about 0.01 to 100. The '513 patent discloses that the resultant cyclohexylbenzene can then be oxidized to the corresponding hydroperoxide and the peroxide decomposed to the desired phenol and cyclohexanone.
However, although benzene hydroalkylation is an attractive route for the production of cyclohexylbenzene, with current processes the selectivity to the desired cyclohexylbenzene product at conversions above 30% is generally less than 70%. The major impurities in the product are cyclohexane and dicyclohexylbenzene. Cyclohexane builds up in the C6 recycle stream and must be removed by treatment or purging, whereas the dicyclohexylbenzene by-product requires transalkylation. Although transalkylation of dicyclohexylbenzene with benzene produces additional cyclohexylbenzene product, the cost of transalkylation is not insignificant. There is therefore a need to provide a benzene hydroalkylation process with improved selectivity to monocyclohexylbenzene.
U.S. Pat. No. 3,784,617 discloses a process for the hydroalkylation of mononuclear aromatic compounds, in which an aromatic charge and a first portion of hydrogen are reacted in a first stage to produce a partially hydroalkylated stream and, after cooling, the partially hydroalkylated stream and a second portion of hydrogen are reacted in a second stage to produce a hydroalkylate product. Introducing the hydrogen in multiple stages reduces the degree of benzene conversion, and hence the temperature rise, in each stage. By avoiding excessive temperature increases, more favorable product selectivities are said to be obtained.
According to the present invention, it has now been found that cooling and recycling of part of the effluent from the hydroalkylation reaction zone, with or without staged addition of hydrogen, allows improved temperature control such that the hydroalkylation process can be operated at or near isothermal conditions and the selectivity to the desired monocyclohexylbenzene can be maximized.